Staphylococcus aureus, often referred to simply as “staph,” is a bacterium commonly carried on the skin or in the nose of healthy people. Staph bacteria are one of the most common causes of skin infections in the United States. Most of these skin infections are minor (such as pimples and boils) and can be treated without antibiotics. However, staph bacteria also can cause serious infections, such as surgical wound infections, bloodstream infections, and pneumonia.
Staphylococci are Gram-positive spherical bacteria that occur in microscopic clusters resembling grapes. In 1884, Rosenbach described the two pigmented colony types of staphylococci and proposed the appropriate nomenclature: Staphylococcus aureus (yellow) and Staphylococcus albus (white). The latter species is now named Staphylococcus epidermidis. Staphylococci are facultative anaerobes that grow by aerobic respiration or by fermentation that yields principally lactic acid. The bacteria are catalase-positive and oxidase negative. S. aureus can grow at a temperature range of 15 to 45 degrees and at NaCl concentrations as high as 15 percent.
Some staph bacteria are resistant to antibiotics. Methicillin-Resistant Staphylococcus Aureus (MRSA) is a type of staph that is resistant to antibiotics called beta-lactams. Beta-lactam antibiotics include methicillin and other more common antibiotics such as oxacillin, penicillin and amoxicillin. The majority of MRSA infections occur among patients in hospitals or other healthcare settings (referred to as hospital-acquired MRSA or HA-MRSA infections); however, Staph and MRSA can also cause illness in persons outside of hospitals and healthcare facilities. MRSA infections that are acquired by persons who have not been recently (within the past year) hospitalized or have had a medical procedure (such as dialysis, surgery, catheters) are known as community-acquired MRSA (CA-MRSA) infections. About 75 percent of CA-MRSA infections are localized to skin and soft tissue and usually can be treated effectively. However, CA-MRSA strains display enhanced virulence, spread more rapidly and cause more severe illness than traditional HA-MRSA infections, and can affect vital organs leading to widespread infection (sepsis), toxic shock syndrome and pneumonia.
It is not known why some healthy people develop CA-MRSA skin infections that are treatable whereas others infected with the same strain develop severe, fatal infections. Studies have shown that rates of CA-MRSA infection are growing. In 1999, four children in Minnesota and North Dakota were reported to have died from fulminant CA-MRSA infections. One study of children in south Texas found that cases of CA-MRSA increased 14-fold between 1999 and 2001. By 2007, CA-MRSA was the most frequent cause of skin and soft tissue infections seen in emergency departments in the United States.
Hospital strains of S. aureus are usually resistant to a variety of different antibiotics. A few strains are resistant to all clinically useful antibiotics except vancomycin, and vancomycin-resistant strains (VRSA) are increasingly-reported. Methicillin resistance is widespread (MRSA) and most methicillin-resistant strains are also multiple drug-resistant. In addition, S. aureus exhibits resistance to antiseptics and disinfectants, such as quaternary ammonium compounds, which may aid its survival in the hospital environment.
More people in the U.S. now die from MRSA infection than from AIDS. MRSA was responsible for an estimated 94,000 life-threatening infections and 18,650 deaths in 2005, as reported by CDC in the Oct. 17, 2007 issue of The Journal of the American Medical Association. The national estimate is more than double the invasive MRSA prevalence reported five years earlier. That same year, roughly 16,000 people in the U.S. died from AIDS, according to CDC.